Questions tagged [singularity-theory]
Singularities in algebraic/complex/differential geometry and analysis of ODEs/PDEs. Singular spaces, vector fields, etc.
554 questions
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Is there a, in depth, classification of branch points in complex analysis?
Disclaimer: This question was originally posted in math.stackexchange.com and, after 30 days with no answers, I followed the instructions of this topic.
In complex analysis we have well known results ...
- 480
4
votes
2
answers
814
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Are Du Val singularities smoothable?
I am interested in when a Du Val surface singularity is smoothable. By Du Val singularity, I mean (the germ of a) isolated double point surface singularity admitting a resolution by blowups of ...
- 205
1
vote
1
answer
373
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Singularities of Chow varieties
Let $X$ be a smooth complex projective variety. The Chow variety of degree $d$, $r$ dimensional subvarieties is denoted by $C_{d,r}(X)$. The Chow variety can have many topologically connected ...
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votes
2
answers
992
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Gorenstein varieties: why the two definitions are equivalent?
There are two definitions of Gorenstein singularities
in the literature. Using Grothendieck's (or Serre's) duality, one
defines the "dualizing sheaf" an object $\hat K_M$ of derived category
...
- 9,237
1
vote
0
answers
27
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What is the relation between the different codimensions (e.g. left-right, contact) of map germs?
I would like to clarify the relations between the different codimension conceptions of map germs. I studied mostly from the new book of Mond and Nuno-Ballesteros, another source is Wall. Most of my ...
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4
votes
1
answer
331
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Is the pull-back of canonical sheaf invertible (modulo torsion)?
Let $X$ be a $\mathbb{Q}$-Gorenstein (isolated) singularity of dimension at least $2$ and $f:Y \to X$ be a resolution of singularities. In this case the canonical sheaf $K_X$ is not necessarily ...
- 2,323
2
votes
1
answer
218
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Existence of terminal $3$-fold flips
Does there exist a terminal $3$-fold $X$ with a curve $C\subset X$ such that $K_X\cdot C < 0$ admitting a Mori flip $X\dashrightarrow Y$, flipping $C$ to a curve $C'\subset Y$, where the singular ...
- 8,998
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0
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64
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Real (non-complex) Du Val singularities for quartics of high global Milnor number
I posted this in MathStackexchange and was advised to come here. As I am only looking for examples, I didn't feel MathOverflow was necessary.
I am looking for examples of specific quartic projective ...
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13
votes
1
answer
685
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Does a resolution of a rational singularity have rationally connected fibers?
A rational singularity is a singularity of a
complex variety $X$ such that for any
resolution $\pi:\; \tilde X\rightarrow X$ the
higher direct images $R^i\pi_*(O_{\tilde X})$
vanish for all $i>0$. ...
- 9,237
3
votes
0
answers
133
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Tensor product by the canonical module preserves Cohen-Macaulayness
Let $X$ be a $\mathbb{Q}$-Gorenstein variety of dimension at least $2$. Suppose that $X$ is normal and Cohen-Macaulay with at worst isolated singularities. Let $F$ be a maximal Cohen-Macaulay $\...
- 2,323
6
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0
answers
96
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forms on singular spaces that can be integrated on an LCI
I'd like to characterize rational $k$-forms on a singular scheme $X$ which can be integrated on any (real) bounded submanifold that is locally LCI in $X$ in a real sense (i.e., which is the real ...
- 7,972
2
votes
0
answers
266
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Why does nearby cycles of a local system on $\mathbb{G}_m$ have same monodromy as local system?
Apologies if this belongs on MSE, but none of the tags I wanted existed, so I took it as a hint to post on MO.
Edit: here is my definition of nearby cycles. Suppose $X$ is a complex analytic space ...
- 272
5
votes
0
answers
185
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How to judge whether an orbifold is good
My own case comes from dynamic system on compact complex manifolds. To be precise, let $M$ be a compact complex 3-dimentional manifold, $W^c$ a holomorphic foliation of M with 1-dimentional uniformly ...
- 661
4
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1
answer
498
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Does the quotient of a variety with log terminal singularities also have log terminal singularities?
Boutot's theorem says that if $X$ is a variety over a field of characteristic 0 with rational singularities, and if $G$ is a reductive group acting on $X$, then the quotient $X/G$ has rational ...
- 93
4
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1
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282
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Is pseudo-rationality preserved by etale morphisms?
Let $f: Y \to X$ be an etale morphism of schemes.
If $X$ has pseudo-rational singularities then does $Y$ also have pseudo-rational singularities?
For the definition of pseudo-rational see, for ...
- 10.5k
4
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2
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204
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Newton polygon notation for algebraic surface singularities
In various sources (e.g. here, Theorem 1.1 and here, Theorem 2.1 (3)), a certain notation which uses a fraction followed by a tuple is used to describe surface singularities. For example, the first ...
- 49
3
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0
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150
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How to distinguish the singularities on moduli space?
Let me start with concrete examples. Let $X$ be a smooth special Gushel-Mukai threefold and $\mathcal{C}(X)$ be its honest Fano surface of conics, it has two irreducible components $\mathcal{C}(X)=\...
- 1,982
6
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1
answer
276
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How to solve the following ODE with a parameter?
I am considering the following ODE
\begin{equation}
\begin{split}
&\frac{d^2}{dy^2}u + \frac{\alpha}{(1+y^2)^{\frac{r}{2}}}u = \delta(y)\\
&\lim_{|y|\to \infty}u(y) = 0.
\end{split}
\end{...
- 903
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0
answers
398
views
A criterion for rational singularities in mixed characteristic
Let $R$ be a mixed characteristic discrete valuation ring with perfect residue field and $f:X \to \mathrm{Spec}(R)$ a flat proper morphism.
If the generic fibre of $f$ is smooth and the special fibre ...
- 10.5k
4
votes
0
answers
367
views
Relationship between exact symplectomorphisms and Hamiltonian diffeomorphisms (in the context of Milnor fibers)
Here is a preamble/setup. Suppose we have a symplectic manifold $(M,d\lambda)$ with an exact symplectic form. By Stokes theorem, $M$ must have nonempty boundary. An exact symplectomorphism $\phi:M \to ...
- 508
3
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0
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275
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Monodromy group of the Milnor fiber of an ADE surface singularity
Let $X$ be a hypersurfaces in $\mathbb C^3$ defined by $f(x,y,z)=0$ with an ADE type singularity at $0$. Denote $\mu$ the Milnor number of the singularity.
On the one hand, we can fit $(X,0)$ into the ...
- 1,803
3
votes
1
answer
191
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A convolution type singular integral operator with log
Define a convolution type operator $T_m$ by
$$T_m(f) = p.v.\int_\mathbb{R}f(x-y)\frac{\log^m|y|}{y}dy.$$ Here $m\ge0$ is an integer.
Consider $f \in H^s (s > 0)$ which is the usual Sobolev space. ...
- 903
8
votes
1
answer
273
views
Is there a "minimal" Whitney stratification of a complex hypersurface?
Let $X\subset \mathbb C^n$ be a complex hypersurface (given by $F=0$ where $F$ is a polynomial). It is known then that $X$ admits a Whitney stratification. This is a decomposition of $X$ into smooth ...
- 14.3k
2
votes
0
answers
62
views
Whitney stratification of a $\mathbb C^*$-invariant hypersuface in $\mathbb C^n$
Let $H\subset \mathbb C^n$ be an irreducible hypersurface invariant under a diagonal $\mathbb C^*$-action with positive weights ($H$ is given by a quasi-homogeneous polynomial). Consider the Whitney ...
- 14.3k
5
votes
1
answer
355
views
Computing the invariants of ball quotient surfaces
The two-dimensional complex unit ball $B$ has group of biholomorphic automorphisms $PU(2,1)$.
If $Γ$ is an arithmetic subgroup of $PU(2,1)$, the quotient $Γ\text{\\}B$ is an orbifold.
Taking its ...
- 9,501
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1
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571
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When is a birational bijection étale?
So this question is probably not "research level", although, for what it is worth, it is coming up in a research paper I am presently writing.
Let $X,Y$ be irreducible affine varieties over $...
- 8,529
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0
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26
views
Question of terminology concerning singularities of transversal type A2
If we consider a plane curve that is a Legendrian front with a singularity of type A2, we say that this singularity is a cusp. If we consider a surface of (for instance) R^3 that is a Legendrian front ...
- 51
6
votes
0
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234
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Resolution graph of higher dimensional ADE singularities
I am looking for different configurations of the exceptional divisors arising from blowing up a higher dimensional ADE singularity (see p. 240 of this article of Bruns for a description of such ...
- 1,981
4
votes
1
answer
282
views
Higher direct image for rational singularities
Let $X$ be a normal, projective (complex) variety with at worst rational singularities. Let $\pi:Y \to X$ be the resolution of singularities obtained by blowing-up the singular points. Is $R^1 \pi_*\...
- 2,323
2
votes
1
answer
215
views
Bertini type result for complete intersection varieties containg a non-singular variety
Let $X$ be a smooth, projective $\mathbb{C}$-variety of dimension $r$ containing in $\mathbb{P}^n$. Denote by $I_X \subset \mathbb{C}[X_0,...,X_n]$ the ideal of $X$ defined by some homogeneous ...
- 2,323
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0
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91
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Maslov cycle for the Conley-Zehnder index - what are its regular points?
I'm looking at the definition of the Conley-Zehnder index, where it is important to look at the group $$\text{Sp}(2n)^* := \{ A \in \text{Sp}(2n) | \det (A-\text{Id}) \neq 0 \}$$and its complement $$\...
- 281
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0
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67
views
Robust features intuition?
The terminology robust features was introduced by Ian Porteous as they are features of a surface wich be followed when the surface is deformed. They capture important aspects of the surface geometry. ...
- 119
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0
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135
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On resolution of singularities over an Artin ring
For a locally noetherian scheme $X$, Grothendieck conjectured that if $X$ is quasi-excellent then there is a proper birational map $Y \to X$ s.t. $Y$ is regular.
We now fix an Artin ring $R$ whose ...
- 519
5
votes
1
answer
359
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Computations of divisor class monoids
Let me first recall some definitions from the very first pages of Bourbaki, Commutative Algebra, Chapter 7, "Divisors".
Let $A$ be a (commutative) domain, $K$ its field of fractions. A ...
- 26.1k
4
votes
2
answers
463
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restricting the "Whitney" map
$\newcommand\R{\mathbb R}$Suppose $f:\R^2 \to \R^2$ is a Whitney map with singularities (well, I'm not sure if this is the name for it, Whitney calls them excellent maps in his 1955 paper), i.e. it is ...
- 529
2
votes
0
answers
191
views
Blowing up the zero section for "Chasse au Canard" (some new kind of geometric canards)
In this paper "Canard cycles and center manifolds" one encounters the blowing up of a non isolated set or manifold of singularities of a vector field or a singular foliation. This is a ...
- 366
3
votes
1
answer
199
views
Is it possible that $\int |f^2(z)|^{t+1}(P\bar{P})\phi(z) dz=0$ for all compactly supported $\phi$?
When proving that the log-canonical threshold is minus the largest root of the Bernstein-Sato polynomial one considers the integral
$$\int |f^2(z)|^{t+1}(P\bar{P}(\phi(z))) dz,$$
where $P$ is a linear ...
- 2,788
8
votes
2
answers
387
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Milnor lattice and Du Val singularity
I am reading this paper: https://arxiv.org/abs/0810.2687 by A. J. de Jong and Robert Friedman. In the proof of Theorem 4.10, a singularity of the following type shows up $$y^2=x^3+z^{6d-1}.$$ When $d=...
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0
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148
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Does every sequence of deformation of singularities eventually become equisingular?
Suppose we are over a field of characteristic zero and $f_i\colon X_i\to \mathrm{Spec}(R_i)$ $(i=1,2,\cdots)$ are flat families of singularities over DVRs. Assume that the generic fiber of $f_i$ is ...
- 131
1
vote
1
answer
742
views
Non-transverse intersection of submanifolds
What can we tell about non-transverse intersection points of (smooth) submanifolds?
Especially, in the case of complementary dimensions, how can we define and calculate the ''multiplicity'' of an ...
- 194
1
vote
1
answer
207
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Numerical methods for evaluating singular integrals
The Helmholtz decomposition for a vector field B contains both volume integrals and two boundary integrals (https://en.wikipedia.org/wiki/Helmholtz_decomposition). For brevity I show just one of the ...
- 11
3
votes
1
answer
217
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Weak Fano varieties and small transformations
A projective normal and $\mathbb{Q}$-factorial variety $X$ is said to be log Fano if there exists and effective divisor $D$ on $X$ such $-K_X-D$ is ample and the pair $(X,D)$ is klt.
Now, let $f:X\...
user168611
4
votes
1
answer
503
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singular metric (with essential singularity)
Working on some $Q$-curvature equation in dimension $4$, I have been faced with singular metric of the form $(\mathbb{B}, e^{-1/\vert x\vert ^2} \vert dx\vert)$. I try to figure out to what those ...
- 914
3
votes
0
answers
199
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Divisorial contractions and singularities
I have a smooth $6$-fold $X\subset\mathbb{P}^n$ and a divisor $D\subset X$ cut out by a quadratic polynomial. I know that $D$ in singular along a smooth $3$-fold $Y\subset X$, and that if $Z$ is the ...
user125056
2
votes
1
answer
42
views
Lifting of control data along a stratified morphism
Let $f:X\to Y$ be a stratified map between Whitney stratified spaces such that for each stratum $S$ of $Y$, $f:f^{-1}(S)\to S$ is a proper stratified submersion. Let $\mathscr{T}_Y$ be a Thom–Mather ...
- 1,102
1
vote
0
answers
30
views
Stratification which makes the defining functions isotrivial
Let $0\in X\subset\mathbb{C}^N$ be a germ of complex space and $0\in Z\subset X$ be a closed analytic subset (globally) defined by holomorphic functions $f_1,\dots,f_r$. Is there a complex analytic ...
- 1,102
5
votes
0
answers
166
views
Normal singularities homeomorphic to a smooth space
I am looking for examples of normal complex spaces $X$ which locally around a singular point are homeomorphic to a smooth complex manifold.
The only example I know is a curve with a cusp, but this is ...
- 2,384
3
votes
1
answer
214
views
Holomorphic vector fields with a non-degenerate isolated zero
Let $v$ be a holomorphic vector field defined in a neighbourhood of $0$ on $\mathbb C^n$ with an isolated zero at $0$. Let $\sum_{i,j}{a_{ij}}z_i\frac{\partial}{\partial z_j}$ be the linear term of $...
- 14.3k
13
votes
1
answer
766
views
J.-P. Serre: Duality of regular differentials on singular curves
I already asked this on math.stackexchange.com, but didn't get any responses. I hope it is appropriate here.
Let $X'$ be an irreducible singular algebraic curve over an algebraically closed field $k$, ...
- 1,286
2
votes
0
answers
237
views
Singularity of L^1-solutions to elliptic PDEs on the puntured ball
Let $\mathbb{B}$ be the unit ball in $\mathbb{R}^n$. Then it is true that if $u\in L^1(\mathbb{B})$ such that $\Delta(u)=0$ on $\mathbb{B}\backslash\{0\}$, then $\Delta(u)$, as a distribution on $\...
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